1) Field of the Invention
The present invention relates to a rare earth permanent magnet having rare earth elements R, transition metal elements T and boron B as a main composition, which provides excellent magnetic properties.
2) Description of Related Art
Among rare earth permanent magnets, demand for Ndxe2x80x94Fexe2x80x94B system magnets has increased annually because of its excellent magnetic properties and because it is relatively inexpensive due to the abundant resources of Nd. Research and development to enhance the magnetic properties of Ndxe2x80x94Fexe2x80x94B system magnet is being made vigorously. In recent years, a mixing method, wherein various kinds of metal powder and alloy powder of different compositions are mixed and then sintered, has become the main stream in the manufacturing of high performance Ndxe2x80x94Fexe2x80x94B system magnets.
However, since Curie temperature of the Ndxe2x80x94Fexe2x80x94B system magnets is low, its coercive force declines as temperature rises. Various attempts have been made to solve this problem. For example, it has been proposed that adding heavy rare earth elements such as Dy and Tb to the Ndxe2x80x94Fexe2x80x94B system magnets is effective to enhance the coercive force (hereinafter, refer to as xe2x80x9cconventional art Axe2x80x9d).
Also, it has been proposed that appropriately changing the mixing ratio of R-T system alloy powder against R-T-B system alloy powder is effective in enhancing the magnetic properties in a method for manufacturing R-T-B system rare earth permanent magnet that employs a mixing method using a main phase with R2T14B-system intermetallic compound (R is one or more selected from the group of rare earth elements and Y, and T is at least one transition metal element) being a main composition and the R-rich phase being a main composing phase (hereinafter, refer to as xe2x80x9cconventional art Bxe2x80x9d).
Also, it is proposed to add one or more of Ti, Ni, Bi, V, Nb, Ta, Cr, Mo, W, Mn, Al, Sb, Ge, Sn, Zr, Hf, Cu, Si and P (herein after referred to xe2x80x9cTi, etc.xe2x80x9d) in order to enhance the magnetic properties of rare earth permanent magnet (hereinafter, refer to as xe2x80x9cconventional art Cxe2x80x9d).
However, in the conventional art A, the addition of heavy rare earth elements such as Dy and Tb would enhance the coercive force but lower its residual magnetic flux density. Also, heavy rare earth elements are costly compared with other elements. Therefore, the key to the manufacturing cost reduction of rare earth permanent magnets is to decrease the volume of heavy rare earth element additives.
Moreover, rare earth permanent magnets manufactured in the conventional art B had a problem in that while they show a high residual magnetic flux density, the coercive force was low.
Moreover, it has been proposed in the conventional art C that Ti, etc. should be used as additives. However, no element has been specified for realizing both excellent coercive force and residual magnetic flux density.
In view of the above, the present invention provides a rare earth permanent magnet that excels in both coercive force and residual magnetic flux density.
Various studies were made by the present inventors to obtain high magnetic properties. As a result, it was discovered that Bi was effective in enhancing the magnetic properties of rare earth permanent magnets. In particular, when the Bi content is 0.01 to 0.2 wt % in the sintered magnet, it is possible to obtain a rare earth permanent magnet with excellent coercive force and residual magnetic flux density. Therefore, the present invention provides to a rare earth permanent magnet that essentially consists of 20-40 wt % of rare earth element R, 0.5-4.5 wt % of boron B, 0.03-0.5 wt % of M (at least one of Al, Cu, Sn and Ga), 0.01-0.2 wt % of Bi and the balance being at least one transition metal element T.
The rare earth permanent magnet according to the present invention may preferably contain 31-32.5 wt % of Nd+Dy, 0.5-1.5 wt % of boron B, 0.15 wt % or less (but not 0 wt %) of Cu, 0.15-0.3 wt % of Al, 2 wt % or less of Co (but not 0 wt %), 0.01-0.2 wt % of Bi, and Fe as the balance. Also, the Bi content may preferably be 0.02-0.1 wt %. Moreover, the Dy content may preferably be between 2 wt % and 15 wt %.
The rare earth permanent magnet according to the present invention produces excellent magnetic properties of 1.25 T or greater in residual magnetic flux density and coercive force of 1,650 kA/m or greater. In the present invention, it is desirable that Bi is dispersed in the grain boundary phase.
In the present invention described above, the content for M (at least one of Al, Cu, Sn and Ga) may be 0.03-0.5 wt %, and the Bi content be 0.01-0.2 wt %. However, only Bi, whose content be 0.01-0.2 wt %, is also effective without containing M.
Therefore, the present invention also provides to a rare earth permanent magnet may include 20-40 wt % of R, 0.5-4.5 wt % of boron B, 0.01-0.2 wt % of Bi and the balance being at least one transition metal element T.
The rare earth permanent magnet in accordance with the present invention presents excellent magnetic properties of 2,100 or greater (Txc3x97kA/m) in terms of the product (Brxc3x97Hcj) of residual magnetic flux density Br and coercive force Hcj. Also, the value obtained by dividing the coercive force Hcj by the weight percentage of the heavy rare earth element (Hcj/weight percentage of heavy rare earth element) is 230 or greater (kA/mxc3x971/wt %). Therefore, according to the present invention, a rare earth permanent magnet with excellent magnetic properties can be obtained while reducing the amount of costly heavy rare earth element to be added. Here, as for heavy rare earth element, at least one element is selected from the group of Gd, Tb, Dy, Ho, Er, Yb and Lu.
Moreover, the salient point of the present invention is the effect of enhancing coercive force Hcj by adding a small amount of Bi. The value obtained by dividing the coercive force Hcj by the weight percentage of Bi (Hcj/weight percentage of Bi) is 8,000 or greater (kA/mxc3x971/wt %).
Also, the present invention provides a rare earth permanent magnet comprising of a R2T14B magnetic phase and a non-magnetic grain boundary phase wherein Bi is dispersed, with a value obtained by dividing the coercive force Hcj by the weight percentage of Bi (Hcj/weight percentage of Bi) being 8,000 or greater (kA/mxc3x971/wt %).
Rare earth permanent magnets of the present invention described above are suitably applicable to sintered magnets.
Other objects, features and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings.